1. Field of the Invention
The present invention relates to microwave ovens, and, more particularly, to an electronic control system which may be used with microwave ovens and is particularly adapted for use in connection with ovens for restaurants, homes, or in vending apparatus. The present invention also relates to a method whereby such electronic control system may be employed in preparing food items for subsequent rethermalization or reheating on a fully automatic basis.
2. Description of the Prior Art
Various arrangements have been heretofore proposed for controlling the application of microwave energy to microwave ovens in the reheating or rethermalization of food items placed therein. One such arrangement is shown in my prior U.S. Pat. No. 3,854,022 issued Dec. 10, 1974 and assigned to the same assignee as the present invention, wherein a ferrite sensing element is provided on a tray carrying a number of food items which is placed in the microwave oven. The presence of a ferrite sensing element within the oven is detected by an arrangement which responds to the change from a ferromagnetic to a paramagnetic state when the sensing element has absorbed a predetermined amount of energy, by automatically terminating the application of power to the microwave oven.
While such an arrangement is suitable for its intended purpose and is particularly adapted for use in the institutional feeding field where such trays may readily be employed, in certain instances it is desirable to provide an arrangement which would not require a separate sensing element associated with each food item or tray of food items. Such an arrangement would have particular application for ovens used in restaurants, home or for vending apparatus employing microwave oven heating of food items. An arrangement is also disclosed in U.S. Pat. No. 3,875,361 which proposes to mount an energy absorbing member in the heating chamber which is in contact with a temperature sensing element to turn off the oven when the energy absorbing member has been heated to a predetermined temperature. However, such an arrangement suffers from the disadvantage that the energy absorbing member must be cooled down to the same starting temperature before the next heating operation can be performed.
Certain other microwave oven control arrangements have employed an essentially manual timing arrangement wherein a timing mechanism is manually adjusted to control the length of time microwave energy is applied to the food item. Such setting of the manual timer usually takes into account the size of the load, its water content and initial temperature and, based on experience an appropriate setting is selected.
In Schroeder U.S. Pat. No. 2,744,990 a timer control is initially set in accordance with the amount of microwave energy it is desired for the roast to absorb based on its gross weight. The Schroeder patent also proposes to measure the net microwave power absorbed by the roast by subtracting signals proportional to the forward and reflected microwave energy adjacent the microwave generator and employs this signal to drive the timer motor. While such an arrangement may provide some compensation for the shape of the food item, it does not provide automatic compensation for size or number of food items within the microwave oven cavity, nor does it eliminate the need for manual setting of the oven control.
Certain other arrangements have proposed the selective shielding of different food items on a tray so that all items on the tray will arrive at the desired temperature at the same time. Such arrangements are shown, for example, in Stevenson U.S. Pat. Nos. 3,547,661 and 3,615,713. However, such selectively shielded tray arrangements are not suitable for restaurants, home or vending use where, in many instances, a single food item is to be placed in the oven at one time. Furthermore, the control arrangements employed with such selectively shielded arrangements have generally employed a conventional manual timer which must be set to a different value for each tray of food items depending upon the shape, size, weight, etc. of the food items thereon.
It is, therefore, an object of the present invention to provide a new and improved electronic microwave oven control system which avoids one or more of the disadvantages of the above discussed prior art arrangements.
It is another object of the present invention to provide a new and improved electronic control system for a microwave oven wherein an electrical signal proportional to the strength of the electromagnetic field within the cavity is integrated to provide a ramp type signal which is then compared with a reference voltage so as to control the supply of microwave power to the cavity in accordance with the setting of the reference voltage.
It is a further object of the present invention to provide a new and improved electronic control system for a microwave oven which employs a crystal detector positioned to detect the level of electromagnetic field within the cavity when an item to be heated is positioned therein, said level varying inversely with the energy absorbed by the food items to be heated so that the time interval over which microwave energy is supplied to the cavity is automatically varied without requiring manual timing adjustments.
It is another object of the present invention to provide a new and improved electronic control system for a microwave oven wherein the time period over which microwave energy is supplied to the oven is controlled by a device which measures the strength of the electromagnetic field at a particular place within the cavity and linearity compensation is provided for rendering the output signal of this device more nearly proportional with respect to variations in the size of the load placed within the cavity.
Briefly, in accordance with one aspect of the invention, a crystal detector unit is positioned to develop an electrical signal proportional to the level of the electromagnetic field within the microwave oven when a food item to be heated is placed therein and this electrical signal is integrated to provide a ramp type electrical signal which is then compared with a fixed reference signal. Since the level of the electromagnetic field within the cavity varies with the size and number of food items which are being heated at the same time, the time required for the ramp type electrical signal to equal the reference signal will be automatically varied so that the food items are automatically brought to the desired condition at the time the microwave power is terminated. Integration of this electrical signal is accomplished by charging a capacitor at a rate proportional to the amplitude of the electrical signal and this capacitor starts discharging when the integrated signal equals the reference signal and microwave power is removed from the cavity. In accordance with a further aspect of the invention, the capacitor is then recharged after it has been discharged a predetermined amount so as to provide a second heating cycle following an equalizing cycle during which the microwave power is turned off. In accordance with another aspect of the invention, the charging rate of the integrating capacitor can be selectively varied to provide compensation for the initial temperature of the food item placed in the oven.
In accordance with a further aspect of the invention the electronic microwave oven control system may be employed to provide factory timed food items, by properly shaping and/or packaging of the food items so that all of the food items reach their desired but typically different temperatures at the same triggering level or setting of the reference voltage in the electronic control system. With this arrangement, the automatically timed food items may then be automatically brought to their desired temperatures by simply placing them in a remote oven, which may be in a restaurant, a home, or in vending apparatus, which is equipped with a similar electronic control system having the same triggering level adjustment as that employed in the factory timing operation.